This invention relates to satellite communications and more particularly relates to such communications employing optical beams, such as laser beams.
The beams used for space-to-space and space-to-ground optical communications have extremely narrow beam widths that require high bandwidth, closed loop control for pointing and tracking to maintain adequate signal power for communications. The beam widths are so narrow (on the order of 1-20 microradians) that methods are needed to initially acquire the communications beams from the usual 0.1-0.3 degree pointing knowledge uncertainty of current spacecraft. The acquisition method must be highly robust and minimize total weight and power requirements for the optical communications terminal.
Beam acquisition methods have been described in the past. For example, in columns 9-11 and FIG. 5, U.S. Pat. No. 3,504,182 (Pizzurro et al., issued Mar. 31, 1970) describes an acquisition method in which a first beam of a first satellite dwells at one point in a field of view while a second beam of a second satellite scans the entire field of view. When the beams illuminate their respective satellites, the acquisition terminates.
U.S. Pat. No. 3,511,998 (Smokler, issued May 12, 1970) describes an acquisition method employing slow oscillatory scan motion limited by limit switches. Receipt of a second beam signal during the slow scan motion terminates the acquisition (Column 11).
U.S. Pat. No. 5,060,304 (Solinsky, issued Oct. 22, 1991) describes an acquisition method relying on beam reflection (Abstract).
U.S. Pat. No. 5,282,073 (Defour et al., issued Jan. 25, 1994) describes an acquisition method in which the width of the beam is altered during acquisition (Columns 5-6).
U.S. Pat. No. 5,475,520 (Wissinger, issued Dec. 12, 1995) describes an acquisition method in which multiple transmitted beams are defocused to provide wide area coverage during acquisition (Column 2).
U.S. Pat. No. 5,592,320 (Wissinger, issued Jan. 7, 1997) describes an acquisition method in which a beam is modulated with time or location information during the acquisition (Column 3).
U.S. Pat. No. 5,710,652 (Bloom et al., issued Jan. 20, 1998) describes an acquisition system employing an array of a CCD acquisition camera (Column 5).
Each of these prior methods and systems have limitations which decrease its usefulness.
One apparatus embodiment of the invention is useful in a communication system employing an optical beam suitable for transmission of data between a first terminal located on an earth orbiting satellite and a second terminal remote from the first terminal. Apparatus on the first terminal aligns the beam received from the second terminal with a beam receptor located on the first terminal within a first uncertainty region which allows tracking of the beam with sufficient accuracy to enable communication of data with the optical beam. According to one embodiment of the invention, the apparatus may include optics enabling receipt of the beam and transmission of the beam along a path. A positioning mechanism is used to point the optics to the location of the second terminal within a second uncertainty region larger than the first uncertainty region. An acquisition sensor receives at least a portion of the beam and locates the second terminal within a third uncertainty region larger than the first uncertainty region and smaller than the second uncertainty region. A controller is responsive to the locating of the second terminal to cause the positioning mechanism to move at least a portion of the optics to successively adjust the position of the beam path relative to the acquisition sensor whereby the size of the third uncertainty region successively approaches the size of the first uncertainty region to facilitate the commencement of tracking of the beam.
A second apparatus embodiment of the invention is useful in a communication system employing an optical beam suitable for transmission of data between a first terminal located on an earth orbiting satellite and a second terminal remote from the first terminal. Apparatus on the first terminal aligns the beam received from the second terminal with a beam receptor located on the first terminal to enable tracking of the beam with sufficient accuracy to enable communication of data with the beam. The apparatus preferably comprises optics receiving the beam and defining a first field of view with a first center point. A positioning mechanism points the optics in the direction of the second terminal. An acquisition sensor defines a second field of view with a second center point receiving at least a portion of the beam and locating the second terminal within a portion of the first field of view. A controller responsive to the locating of the second terminal by the acquisition sensor controls the positioning mechanism successively to point the first and second center points toward a region represented by the portion of the first field of view in which the second terminal is determined to be located until at least a portion of the beam is sufficiently aligned with the beam receptor to enable tracking.
A third apparatus embodiment of the invention is useful in a communication system employing a first optical beam and a second optical beam suitable for transmission of data between a first terminal located on an earth orbiting satellite and a second terminal remote from the first terminal. Apparatus on the first terminal transmits the first beam from the first terminal for alignment with a second beam receptor located on the second terminal and aligns the second beam transmitted by the second terminal with a first beam receptor located on the first terminal within a first uncertainty region which allows tracking of the second beam with sufficient accuracy to enable communication of data with the second beam. Optics in the first terminal enable transmission of the first beam and receipt of the second beam. The optics preferably comprise a beam deflector scanning the first beam over a controlled second uncertainty region with a first scan pattern defining a first locus of scan lines having a center scan line. A positioning mechanism points the optics toward the location of the second terminal and moves the beam deflector. An acquisition sensor in the first terminal receives at least a portion of the second beam and determines the location of the second terminal within a third uncertainty region. A controller in the first terminal successively controls the positioning mechanism in response to the location determined by the acquisition sensor to cause the beam deflector successively to point the center scan line into the third uncertainty region to facilitate the commencement of tracking.
A fourth apparatus embodiment of the invention is useful in a communication system employing a first optical beam and a second optical beam suitable for transmission of data between a first terminal located on an earth orbiting satellite and a second terminal remote from the first terminal. Apparatus on the first terminal transmits the first beam from the first terminal for alignment with a second beam receptor located on the second terminal and aligns the second beam transmitted by the second terminal with a first beam receptor located on the first terminal to enable tracking of the second beam with sufficient accuracy to enable communication of data with the second beam. Optics in the first terminal defines a first field of view having a first center point enabling receipt of the second beam. The optics preferably comprises a beam deflector scanning the first beam over a second field of view having a second center point within the first field of view. A positioning mechanism points the optics toward the location of the second terminal and moves the beam deflector. An acquisition sensor defines a third field of view with a third center point in the first terminal receiving at least a portion of the second beam and determines the location of the second terminal within a portion of the third field of view. A controller in the first terminal successively controls the positioning mechanism in response to the location determined by the acquisition sensor to cause the positioning mechanism successively to point the first and second center points toward a region represented by the portion of the third field of view in which the second terminal is located to facilitate the commencement of tracking.
A fifth apparatus embodiment of the invention is useful in a communication system employing a first optical beam and a second optical beam suitable for transmission of data between a first terminal located on an earth orbiting satellite and a second terminal remote from the first terminal. Apparatus on the first terminal transmits the first beam from the first terminal for alignment with a second beam receptor located on the second terminal and aligns and tracks the second beam transmitted by the second terminal with a first beam receptor located on the first terminal with sufficient accuracy to enable communication of data with the second beam. A telescope in the first terminal enables transmission of the first beam and receipt of the second beam at least in part along a common first path. The telescope also defines a first field of view. A movable first beam deflector transmits the first beam to the first path and receives the second beam from the first path. A first beam splitter enables the directing of the first beam from a transmission path onto the first beam deflector and enables the directing of the second beam into a receive path. A track sensor defines a second field of view. A second beam splitter in the receive path directs a predetermined percentage of the beam in the receive path along a track sensor path to the track sensor. An acquisition sensor defines a third field of view. A second beam deflector deflects part of the beam in the receive path along an acquisition sensor path to the acquisition sensor and part of the beam in the receive path onto the beam receptor. First transmission optics transmits the first beam along the transmission path. A movable third beam deflector scans the first beam in a predetermined scan pattern along the transmission path and transmits the scan pattern to the first beam splitter. A controller alters the position of the first beam deflector to adjust the second field of view relative to the track sensor and to adjust the third field of view relative to the acquisition sensor and alters the position of the third beam deflector to adjust the scan pattern to enable tracking to begin.
A first method embodiment of the invention is useful in a communication system employing an optical beam suitable for transmission of data between a first terminal located on an earth orbiting satellite and a second terminal remote from the first terminal. The method aligns the beam received from the second terminal with a beam receptor located on the first terminal and defining a center point within a first uncertainty region which allows tracking of the beam with sufficient accuracy to enable communication of data with the optical beam. The method is accomplished by receiving the beam and transmitting it along a path. The second terminal initially is located within a second uncertainty region larger than the first uncertainty region. The intersection of the beam with the first terminal is detected. The second terminal then is located within a third uncertainty region larger than the first uncertainty region and smaller than the second uncertainty region in response to the detecting. The position of the beam path relative to the beam receptor is successively adjusted in response to the locating of the second terminal within the third uncertainty region whereby the size of the third uncertainty region successively approaches the size of the first uncertainty region to facilitate the commencement of tracking of the beam.
A second method embodiment of the invention is useful in a communication system employing a optical beam suitable for transmission of data between a first terminal located on an earth orbiting satellite and a second terminal remote from the first terminal. The method aligns the beam received from the second terminal with a beam receptor located on the first terminal to enable tracking of the beam with sufficient accuracy to enable communication of data with the beam. The method is accomplished by receiving the beam within a first field of view having a first center point. The occurrence of the beam within the first field of view is detected, and the portion of the first field of view in which the second terminal is located is determined in response to the detecting of the beam within the first field.of view. A second field of view having a second center point is defined in response to locating the second terminal within the portion of the first field of view. The second field receives the beam. The second center point is pointed successively toward a region represented by the portion of the first field of view in which the second terminal is determined to be located until at least a portion of the beam is sufficiently aligned with the beam receptor to enable tracking.
A third method embodiment of the invention is useful in a communication system employing a first optical beam and a second optical beam suitable for transmission of data between a first terminal located on an earth orbiting satellite and a second terminal remote from the first terminal. The method transmits the first beam from the first terminal for alignment with a second beam receptor located on the second terminal and aligns the second beam transmitted by the second terminal with a first beam receptor located on the first terminal within a first uncertainty region which allows tracking of the second beam with sufficient accuracy to enable communication of data with the second beam. The method is accomplished by transmitting the first beam over a controlled second uncertainty region with a first scan pattern defining a first locus of scan lines having a center scan line. The second beam then is received. The location of the second terminal within a third uncertainty region is determined successively in response to receiving the second beam. The center scan line is pointed into the third uncertainty region successively to facilitate the commencement of tracking.
A fourth method embodiment of the invention is useful in a communication system employing a first optical beam and a second optical beam suitable for transmission of data between a first terminal located on an earth orbiting satellite and a second terminal remote from the first terminal. The method transmits the first beam from the first terminal for alignment with a second beam receptor located on the second terminal and aligns the second beam transmitted by the second terminal with a first beam receptor located on the first terminal to enable tracking of the second beam with sufficient accuracy to enable communication of data with the second beam. The method is accomplished by defining a first field of view having a first center point enabling receipt of the second beam. The first beam is transmitted over a second field of view having a second center point within the first field of view. A third field of view with a third center point is defined in the first terminal. At least a portion of the second beam is received. The location of the second terminal within a portion of the third field of view is determined successively. The first and second center points are pointed successively toward a region represented by the portion of the third field of view in which the second terminal is determined to be located to facilitate the commencement of tracking.
A fifth method embodiment of the invention is useful in a communication system employing a first optical beam and a second optical beam suitable for transmission of data between a first terminal located on an earth orbiting satellite and a second terminal remote from the first terminal. The method transmits the first beam from the first terminal for alignment with a second beam receptor located on the second terminal and aligns and tracks the second beam transmitted by the second terminal with a first beam receptor located on the first terminal with sufficient accuracy to enable communication of data with the second beam. The method is accomplished by transmitting the first beam and receiving the second beam at least in part along a common first path in the first terminal. A first field of view for receiving the second beam is defined in the first terminal. The first beam is transmitted to the first path, and the second beam is received from the first path. The second beam is directed into a receive path. The second beam is sensed within a portion of a second field of view along a track sensor path. A predetermined percentage of the beam in the receive path is directed along the track sensor path. The beam is sensed within a portion of a third field of view along an acquisition sensor path. Part of the beam in the receive path is transmitted along the acquisition sensor path and part of the beam in the receive path is transmitted to the beam receptor. The first beam is transmitted along the transmission path, and is scanned in a predetermined scan pattern along the transmission path. The scan pattern is directed from the transmission path to the first path. The second field of view is adjusted relative to the track sensor path, the third field of view is adjusted relative to the acquisition sensor path and the scan pattern is adjusted in response to sensing of the beam within a portion of the third field of view to facilitate the commencement of tracking. By using the foregoing techniques, terminal weight and power can be minimized. For example, a transmit laser source communication beam also can be used for acquisition. There is no need for a separate beacon or beam. The foregoing techniques do not require separate laser sources or mechanisms to provide a wide beam or shaped acquisition beam or beacon. The acquisition quadrant sensor used in the preferred embodiment minimizes electronics cost and power. The time in each stage, the number of stages, and the criteria to transition between stages can be optimized to minimize the acquisition time for a particular application. The ability to acquire and reacquire from intermediate stages minimizes the impact of communication outages by minimizing reacquisition time.